In a so-called SCR exhaust gas aftertreatment system (selective catalytic reduction), a liquid reducing agent (normally a urea-water solution, particularly AdBlue) is injected when required into the exhaust gas tract of an internal combustion engine by means of a metering valve; thus enabling a catalytic reduction of nitrogen oxide to take place in an SCR catalytic converter. The urea thereby dissociates to ammonia (NH3), which reacts with the nitrogen oxides (NOx) and converts them into non-toxic water (H2O) and nitrogen (N2).
Electromagnetically controllable metering valves are used in this process, which are actuated by a drive unit. The drive unit includes a magnetic coil, whose magnetic field which is generated by a sufficiently large coil current lifts a valve element of the metering valve from a valve seat and in doing so opens the metering valve. A smaller coil current is necessary to subsequently stop the opened metering valve. The metering valve and the drive unit are thereby combined in a metering module.
It is known that an AdBlue volume expands by approximately 11% during freezing and can consequently cause damage to component parts. The metering valve is generally the most susceptible to damage because it includes very thin-walled and hence pressure sensitive components as, for example, a perforated injection disc. Perforated injection discs are, for example, currently used, which are only approximately 150 μm thick and are therefore particularly subject to compressive forces occurring during freezing.
It is therefore common to empty the metering valve during shutdown of the vehicle's engine, for example, by sucking back the reducing agent. The metering valve is emptied in order to prevent a possible freezing of said valve when frost occurs and also in order to remove the relatively aggressive urea-water solution from the interior of the metering module for reasons of protecting components.
It was determined in tests that a considerable residual amount of AdBlue stays in the metering module in the case of a back suction operation. This residual amount can cause damage upon freezing, a freezing direction being of importance in this case. If freezing occurs in the direction of the perforated injection disc, i.e., for example, emanating from the center of the metering module in the direction of said perforated injection disc, the volume enlargement of AdBlue due to cold also occurs in this direction. That means that said sensitive perforated injection disc is subjected to pressure which can damage the same.
A method for heating a metering module in a SCR exhaust gas aftertreatment system is known from the German patent publication DE 10 2007 017 458 A1, wherein a need for heating the metering module to avoid freezing is ascertained by the evaluation of sensor signals, in particular signals of a temperature sensor, and by their comparison with threshold values. In order to heat said metering module, the closed metering valve is actuated with a current profile, which does not yet lead to the opening of the metering valve and merely releases heat in the ohmic resistor of the coil, which heats up said metering module from the inside out. This current profile is thereby particularly characterized in that its level, i.e. its average amperage, is lower than the level of a first current profile used for metering AdBlue.